Light Emitting Diodes (LEDs) have many advantages over conventional light sources, such as incandescent, halogen and fluorescent lamps. These advantages include longer operating life, lower power consumption, and smaller size. Consequently, conventional light sources are increasingly being replaced with LEDs in traditional lighting applications. As an example, LEDs are currently being used in flashlights, camera flashes, traffic signal lights, automotive taillights and display devices.
There are two main types of circuit arrangements currently used to control/drive LEDs. A first circuit arrangement is a driver circuit which provides constant current to the LED. A second circuit arrangement is a driver circuit which relies on an external resistor to control current to the LED.
Drivers which employ the constant LED current approach are typically based on constant current circuitry in an Integrated Circuit (IC) to determine the amount of LED current flowing through the LED. On the other hand, drivers which employ the external resistor controlled approach require an additional resistor be connected in series with the LED and the resistance of the resistor controls the amount of current which flows through the LED. There are several disadvantages to using either circuit arrangement.
Problems associated with employing the constant LED current approach include: (1) circuit size increases with the increment of LED current selection; (2) higher headroom (or output voltage) is required unless a larger transistor is provided; and (3) there are eye safety issues at low supply levels.
Problems associated with employing the external resistor controlled approach include: (1) a large variation of LED current; (2) extra resistor represents an additional cost; and (3) power saving mode is not available (e.g., it is not possible to switch to lower LED current at brighter surfaces).